Radio Frequency (RF) systems have conventionally employed one or more several means of avoiding interference with other such systems. The most common is broadcast radio, where each broadcaster is allocated exclusive use of a single radio frequency channel and no other transmitters are permitted on that allocated channel. However, in systems using unlicensed radio spectrum, more sophisticated means are often required because no user has exclusive rights to any given frequency. Examples of these methods include Frequency Hopping Spread Spectrum (FHSS), Carrier Sense Multiple Access (CSMA), Time Division Multiple Access (TDMA), Transmit at Will with Retry, and Direct Sequence Spread Spectrum (DSSS).
In FHSS systems, transmitters and receivers “hop” between different radio frequencies in a pre-defined sequence. Such systems have the advantage of almost always being able achieve successful reception of at least some of the transmitted packets, because it is very rare for all of the different frequencies used to be blocked by other radio transmitters. However, they have the disadvantage that frequently at least some of the transmitted packets will not be received if there are other radio systems operating in the same band.
In CSMA systems, a transmitter “listens” for “quiet” on the frequency channel in use before transmitting. In systems where the packet transmission rate is low, CSMA systems have a high probability of receiving transmitted packets. Many CSMA systems are fixed channel systems, but some also use Frequency Agility (described below). A disadvantage of CSMA is that it is generally unsuitable for low-latency systems, as the delay between when a device has data ready to transmit and when the channel is quiet is uncertain.
In TDMA systems, multiple systems may share a single frequency channel, or a single system comprising many transmitters may use the same channel. In a TDMA system, one or more “coordinators” must manage timing for the whole system or group of systems, allocating time slices to each of the other nodes in the system or systems. In this way, each transmitter has an allocated time slice and has exclusive use of the channel during its allocated time slice.
Transmit at Will with Retry systems take a much simpler approach. A transmitter sends its packet whenever data is ready to send. If the data is correctly received, the receiver transmits back a handshake signal. If the transmitter does not receive the handshake signal, it will retransmit the original data packet. By using random backoff times between retransmissions, Transmit at Will with Retry systems can support multiple transmitters. If two (or more) transmitters try to send data at the same time, no packets will be received by the receiver, so no handshake signals will be sent. Both transmitters will then retransmit later, but at different times. DSSS transmission can be combined with this method to improve resistance to packet corruption caused by other RF systems using the same channel and other sources of RF interference.
In DSSS systems, the signal does not hop from one frequency to another but is passed through a spreading function in the transmitter and distributed over a bandwidth greater than the bandwidth of the data rate. In some cases, the entire band may be used to transmit a single channel of data, but, more typically, the band is split into a number of channels each wider than the data rate. A DSSS receiver passes the received signal through a dispreading function, which concentrates the spread transmitted signal and spreads any narrowband interference on the channel. In this way, a DSSS receiver avoids interference by concentrating the desired signal but spreading out and diluting any interfering signal.
Apart from frequency hopping, all of the above methods must have a channel selection method. The simplest such method is for the system to be configured to use a single channel when installed. However, many radio systems use frequency bands where the use of the different channels may vary dynamically. For example, in the 2.4 GHz band, an already present wireless networking (e.g., 802.11) system may not be in use at the time of installation but may later interfere badly when it is in use. Therefore, many such systems employ Frequency Agility.
In a frequency agile system, when persistent interference is detected on the channel currently in use, the system changes to use a different channel with minimal interference. However, one drawback of the conventional implementation of frequency agile systems is that there can be a significant delay in finding a new channel that is free from interference. This delay may result in an interruption of service for tens of milliseconds (ms) or longer.
Thus, an improved frequency agile radio system in which the interruption of service during channel changing is dramatically lower than in conventional systems is desirable.